Sealed containers for perishables, such as food, must be tested to insure the integrity of the seal. Typically, when testing the integrity of such containers, sensors are used in connection with placing the container in an evacuated chamber. The walls of the container bulge in this condition since the pressure is greater inside the container than outside the container (i.e., within the chamber). The sensors then gauge a maximum deflection value of the container wall against a predetermined threshold value. If the package deflection is greater than this value, it is considered to have passed the test. However, if the maximum package deflection is less than this value, it is considered to have failed the test, the reasoning being that the container did not bulge due to a leak. The foregoing test is often referred to as a "bulging wall test." This type of test suffers, however, for several reasons, one of which is that it does not typically measure the leak rate of the container (i.e., the deflection may decrease over the length of the test and go undetected). Several efforts have been made to eliminate this problem. For example, U.S. Pat. No. 3,837,215 discloses measuring the container wall travel and velocity of wall travel, while U.S. Pat. No. 4,715,215 discloses holding the reduced pressure within the enclosure constant for a time interval to determine with the sensor whether the distended portion of the container has relaxed.
The foregoing attempts to solve the problems still suffer, however, in the case of a leak with a diameter of several microns, or a leak which is rapidly plugged by the foodstuff (or other contents of the container). This failure is due to the time interval required for the sensors to record the leak, since the time is burdensome in the former and may be infinite in the latter.
Therefore, there exists a need for a test apparatus and method which measures the deflection of a sealed package placed in an evacuated chamber over a period of time in addition to measuring the status of the vacuum within the evacuated chamber.